problem with bootMer CI: upper and lower limits are identical - lme4

Question

I'm having the hardest time generating confidence intervals for my glmer poisson model. After following several very helpful tutorials (such as https://drewtyre.rbind.io/classes/nres803/week_12/lab_12/) as well as stackoverflow posts, I keep getting very strange results, i.e. the upper and lower limits of the CI are identical.
Here is a reproducible example containing a response variable called "production," a fixed effect called "Treatment_Num" and a random effect called "Genotype":
df1 <- data.frame(production=c(15,12,10,9,6,8,9,5,3,3,2,1,0,0,0,0), Treatment_Num=c(1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4), Genotype=c(1,1,2,2,1,1,2,2,1,1,2,2,1,1,2,2))
#run the glmer model
df1_glmer <- glmer(production ~ Treatment_Num +(1|Genotype),
data = df1, family = poisson(link = "log"))
#make an empty data set to predict from, that contains the explanatory variables but no response
require(magrittr)
df_empty <- df1 %>%
tidyr::expand(Treatment_Num, Genotype)
#create new column containing predictions
df_empty$PopPred <- predict(df1_glmer, newdata = df_empty, type="response",re.form = ~0)
#function for bootMer
myFunc_df1_glmer <- function(mm) {
predict(df1_glmer, newdata = df_empty, type="response",re.form=~0)
}
#run bootMer
require(lme4)
merBoot_df1_glmer <- bootMer(df1_glmer, myFunc_df1_glmer, nsim = 10)
#get confidence intervals out of it
predCL <- t(apply(merBoot_df1_glmer$t, MARGIN = 2, FUN = quantile, probs = c(0.025, 0.975)))
#enter lower and upper limits of confidence interval into df_empty
df_empty$lci <- predCL[, 1]
df_empty$uci <- predCL[, 2]
#when viewing df_empty the problem becomes clear: the lci and uci are identical!
df_empty
Any insights you can give me will be much appreciated!

Answer 1

Ignore my comment!
The issue is with the function you created to pass to bootMer(). You wrote:
myFunc_df1_glmer <- function(mm) {
predict(df1_glmer, newdata = df_empty, type="response",re.form=~0)
}
The argument mm should be a fitted model object derived from the bootstrapped data.
However, you don't pass this object to predict(), but rather the original model
object. If you change the function to:
myFunc_df1_glmer <- function(mm) {
predict(mm, newdata = df_empty, type="response",re.form=~0)
#^^ pass in the object created by bootMer
}
then it works:
> df_empty
# A tibble: 8 x 5
Treatment_Num Genotype PopPred lci uci
<dbl> <dbl> <dbl> <dbl> <dbl>
1 1 1 12.9 9.63 15.7
2 1 2 12.9 9.63 15.7
3 2 1 5.09 3.87 5.89
4 2 2 5.09 3.87 5.89
5 3 1 2.01 1.20 2.46
6 3 2 2.01 1.20 2.46
7 4 1 0.796 0.361 1.14
8 4 2 0.796 0.361 1.14
As an aside -- how many genotypes in your actual data? If less than 5-7 you might
do better using a straight up glm() with genotype as a factor using sum-to-zero
contrasts.